This invention relates to a drill stem testing process (or formation testing process) for determining what fluid, if any, can be produced from a subterranean reservoir which is encountered by a well being drilled. It also relates to a liquid-injection-effected process for gas-lifting liquid from within a well. It is especially useful for relatively remote and/or deep exploratory wells. The present process provides a comparatively quick and inexpensive, but safe, procedure for determining whether any fluid can be produced in response to a selected drawdown within a well; and it can provide a production test which is long enough and strong enough to indicate whether a suitable rate of production is apt to be sustained and/or whether the reservoir is apt to slough or produce sand.
Drill stem testing techniques and equipment have been known and used for more than about 50 years. Numerous improvements in the tools and techniques have been described in patents and publications such as the text book "Petroleum Production Engineering and Oil Field Development" by Uren (1941), U.S. Pat. No. 2,850,097; 3,038,539; 3,059,695; 3,233,676; 3,235,017 and technical journal articles, such as "Simple Field Checks Will Provide Accurate DST Data", World Oil, April 1974, and "Obtain Accurate Data From Deep Formation Tests", World Oil, October 1974, and the like.
In the early reservoir tests it was necessary to remove the drill string and replace it with a testing tool each time a test was to be made. More recently, combined drilling and testing tools were developed to avoid the need for pulling the drill string. But, even with the improved tools, it is necessary to remove enough drilling fluid from the borehole to reduce the hydrostatic pressure to less than the reservoir fluid pressure and provide a drawdown which will induce a fluid inflow from a productive formation. For example, in regions where adequate testing equipment and services are available and it is not essential to use a pressurized gas cushion within the testing tool, reservoir testing tools can be arranged to be run in dry (i.e., so that they are filled with air at atmospheric pressure) and operated so that, after a packer is set, the testing tool valves are opened to expose the formation to the drawdown imposed by a pressure which is initially substantially as low as atmospheric pressure.
Alternatively, where pressurized nitrogen is available, it can be pumped through such a testing tool string to displace the drilling fluid into the annulus. Then, after the packer is set, the gas can be depressurized at a controlled rate in order to initiate the production of fluid from the reservoir. Such a "cushion", comprising a pressurized gas within the test string at a pressure below the expected reservoir pressure, is often believed to be desirable. The cushion may avoid the risk of a dry run, e.g., due to the weight of liquid contained within the test string being excessive; or may avoid a tool-damaging surge of fluid inflow that may be accompanied by cavitation and a blowout, e.g., due to the amount of liquid within the test string being insufficient.
U.S. Pat. No. 3,612,183 describes a process for displacing the drilling fluid from the interior of a combined drilling and testing tool without using a cryogenic fluid such as liquefied nitrogen. The patented process injects a liquefied petroleum gas (e.g. propane) which is liquid at the pressure at which it is injected but becomes a suitably pressurized gas when the pressure is reduced.
During the course of research directed to problems such as the initiating of a flow of fluid from a reservoir in which the fluid pressure is less than the hydrostatic pressure of the liquid in a borehole which encounters the reservoir, initiating a perforation-cleaning pulse of relatively highly pressurized fluid and heat within a low pressure reservoir surrounding a borehole which has been cased and perforated, etc. it was discovered that various organic and/or inorganic reactants could be dissolved in an aqueous liquid to form a gas-generating solution which is capable of generating a significantly large volume of gas at a suitable rate within a borehole or a reservoir. Such gas-generating aqueous liquid solutions are described, in U.S. Pat. No. 4,178,993 by E. A. Richardson and R. F. Scheuerman, for initiating a flow into a well by reducing the hydrostatic pressure within a liquid-containing wellbore. The U.S. Pat. No. 4,178,993 describes injecting a solution of nitrogen gas-forming reactant having a composition and concentration correlated with the pressure, temperature and volume properties of the well and reservoir so that the solution remains substantially unreactive until it reaches a selected depth within the borehole and then begins to generate gaseous nitrogen at a moderately rapid rate.
U.S. Pat. No. 4,219,083 by E. A. Richardson, R. F. Scheuerman and D. C. Berkshire relates to chemically inducing a backsurge of fluid through well casing perforations. It describes injecting a solution of nitrogen gas-forming reactants through the well casing perforations and into the reservoir. The solution used in U.S. Pat. No. 4,219,083 patent comprises the solution of U.S. Pat. No. 4,178,993 patent modified by an addition of a reaction-retarding alkaline buffer and a pH-reducing reactant capable of subsequently overriding that buffer. It was found that such reactants could be arranged so that the solution remains substantially unreactive within the well but, within the reservoir, becomes an acidic, fast-reacting solution which generates a rapid-rising pulse of heat and gas having a pressure sufficient to cause a debris-removing backsurging of fluid through the casing perforations.
We have now discovered that particular ones of such gas-generating reactants can be used in particular ways in conjunction with particular types of drilling and testing tools in a manner which provides an unobviously advantageous process for testing a subterranean reservoir and/or removing liquid from within the borehole of a well.